The present invention relates to an image-forming apparatus such as a laser printer in which electrophotography is used to form an image on print paper, and more particularly to a temperature-controlling apparatus that controls the temperature of a fixing unit that fixes an image transferred onto the print paper.
With an electrophotographic image-forming apparatus, laser light illuminates a photoconductive drum to form an electrostatic latent image thereon. The electrostatic latent image is then developed with toner into a toner image. Then, the toner image is transferred from the photoconductive drum to print paper. Finally, the toner image on the print paper is fixed at the fixing unit and then the paper is discharged from the fixing unit.
FIG. 30 illustrates a fixing unit and the associated control sections of a conventional image-forming apparatus.
The conventional image-forming apparatus will be described with reference to FIG. 30.
A fixing unit 100 includes a heat roller 1, a pressure roller 20, a heater 2, two springs 21, and a temperature detector 30. The heat roller 1 heats a toner image, transferred to a print medium such as A5 size paper 70 or A4 size paper 80, to fix the toner image. The heater 2 generates heat and supplies the heat to the heat roller 1. The pressure roller 20 is rotatably supported by a shaft 22 and rotates in pressure contact with the heat roller 1. The springs 21 are disposed at longitudinal ends of the shaft 22 and urge the shaft 22 against the heat roller 1 such that the pressure roller 20 is in pressure contact with the heat roller 1. The temperature detector 30 is in contact with the outer circumferential surface of the heat roller 1 to detect the temperature of the heat roller 1.
In order to uniformly heat the toner image on the print medium 70 or 80, the heat roller 1 takes the shape of a long, hollow cylinder made of a highly heat conductive metal material. The heat roller 1 has a rubber material formed on an outer circumferential surface of the cylinder with a resin coating on the rubber. The coating is highly heat resistant and has good mold-releasing characteristic. The heat roller 1 and the pressure roller 20 are rotated in directions shown by arrows 90 and 91 of FIG. 30, respectively.
The heater 2 takes the form of, for example, a halogen lamp that extends in the longitudinal direction of the heat roller 1 but is not in contact with the heat roller 1.
The pressure roller 20 is a rubber roller in pressure contact with the heat roller 1. The pressure roller 20 is rotated such that the print medium 70 or 80 is pulled in between the heat roller 1 and the pressure roller 20 and is advanced in a direction shown by arrow 92.
The temperature detector 30 takes the form of, for example, a temperature measuring thermistor. The temperature detector 30 is disposed, on the circumferential surface of the heat roller 1 outside of a paper path in which the A4 size paper (medium 80) passes. In this specification, A4 size paper in portrait orientation is a maximum size of print medium that the fixing unit can accept. Specifically, the temperature detector 30 is a distance L2 (e.g., 0-10 mm) away from the paper path.
A control block 200 shown in FIG. 30 will be described.
A main controller 11 controls the supply of electric power to the heater 2, transport of the print medium 70 or 80, and drive of the pressure roller 20. An analog/digital (A/D) converter 12 that converts temperature data in analog form detected by the temperature detector 30 into a digital form that can be processed in the main controller 11. A power switch 13 that switches on and off the electric power supplied to the heater 2 from a power supply 14 under the control of the main controller 11. In accordance with the instructions from the main controller 11, a drive controller 15 drives the drive source 16 in the form of, for example, a motor that drives the pressure roller 20 in rotation. A memory 17 stores a control program for the main controller 11 and control data. The operating panel 18 includes an input section 18b and a display section 18a. The input section 18b allows the user to set the size of print paper and print density prior to printing. The display section 18a displays current settings and messages to the operator.
The warm-up operation of the aforementioned conventional image-forming apparatus is performed as follows:
For example, upon power up, the warm-up operation starts. The main controller 11 causes the power switch 13 to shift from an OFF position to an ON position, so that the power supply 14 supplies electric power to the heater 2. At the same time, the main controller 11 causes the drive controller 15 to drive the motor 16 into rotation. Thus, the motor 16 drives the pressure roller 20 and heat roller 1 in rotation in the directions shown by arrows 90 and 91. In the warm-up operation, when the temperature detected by the detector 30 reaches a target temperature (e.g., 170xc2x0 C.) at which the fixing operation can be adequately carried out, the supply of electric power from the power supply to the heater 2 is interrupted. Thereafter, feedback control is performed through a loop (the detector 30xe2x86x92A/D converter 12xe2x86x92main controller 11xe2x86x92power switch 13xe2x86x92heater 2xe2x86x92heat roller 1), thereby maintaining the detected temperature to the target temperature.
The operation when the image-forming apparatus receives print data from a host apparatus will be described.
Upon receiving the print data from the host apparatus, the main controller 11 provides a command to the power switch 13 so that the power switch 13 directs electric power to the heater 2 as required. The main controller 11 also provides a command to the drive controller 15, thereby causing the drive controller 15 to drive the motor 16 to drive the pressure roller 20 and heat roller 1 in rotation. Then, the main controller 11 causes the print medium 70 or 80 to be fed to the image-forming section from a paper cassette, not shown. A toner image is transferred onto the print medium 70 or 80 and the print medium is advanced by a registry roller, not shown, in a direction shown by white arrow 92 and abuts the pressure roller 20. The print medium is pulled in between the rotating heat roller 1 and the pressure roller 20. When the print medium passes between the heat roller 1 and pressure roller 20, the toner on the print medium 70 or 80 is melted and permanently adheres to the print medium due to the pressure applied by the pressure roller 20. The print medium 70 or 80 is then discharged with a transport device, not shown, to the outside of the printer.
As described above, the temperature detector 30 is disposed in contact with an area outside of the surface of the heat roller 1 that contacts the print medium. The electric power supplied to the heater 2 is switched on and of f such that a temperature detected by the temperature detector 30 is maintained at a predetermined value.
When the print medium 70 or 80 passes between the heat roller 1 and pressure roller 20, the print medium absorbs heat from the heat roller 1 to fuse the toner deposited thereon, thereby causing the temperature of the heat roller 1 to decrease. This creates a temperature difference between an area on the heat roller 1 that contacts the print medium and an area on the heat roller 1 that does not contact the print medium.
For example, when A4 size paper (print medium 80) passes through an area Lb, the heat roller 1 loses heat to the A4 size paper, creating a temperature difference between the area Lb and a point E. However, the distance L2 between the area Lb and the point E is very short, e.g., 0-10 mm. Thus, the temperature difference between the area Lb and the point E is not large, so that the temperature in the area Lb can be maintained at a predetermined target temperature by feedback control using a temperature detected by the temperature detector 30 in contact with the point E.
Also, when, for example, A5 size paper (print medium 70) smaller than A4 passes through the area 1a, the heat roller 1 loses heat to the A5 size paper, creating a temperature difference between the area 1a and the point E. However, the distance L1 between the area 1a and the point E is much longer than L2. Thus, the heat does not transfer so fast through the heat roller 1, so that a temperature difference between the area 1a and the point E is large. Despite the fact that the temperature at the point E can be controlled to a predetermined target value, the temperature in the area 1a is not sufficient for proper fixing. In other words, when a plurality of pages are printed continuously, the temperature difference increases. Thus, the aforementioned conventional controlling method cannot maintain the proper temperature of the heat roller 1 in direct contact with a narrow-width print medium.
The present invention was made in view of the aforementioned drawbacks of the conventional apparatus.
An object of the invention is to provide an image-forming apparatus for printing on print media having a variety of widths wherein when pages of a small-width print medium are printed continuously, the temperature of the heat roller in contact with the print medium is maintained at a constant temperature.
A fixing unit is used in an image forming apparatus. A heat roller heats a toner image on a print medium. A heater is disposed in the heat roller and supplies heat to the heat roller. A first temperature detector is in the form of a thermistor. The first temperature detector is disposed in contact with a first surface area of an end portion of the heat roller outside of a second surface area of the heat roller with which the print medium passes in contact. The first temperature detector generates a first temperature data. A second temperature detector is disposed to oppose the heat roller in proximity to a substantially middle portion of the second surface, and generates a second temperature data. A controller controls supply of electric power to the heater. Based on the first temperature data and the second temperature data, the controller switches between a first temperature control mode and a second temperature control mode. The first temperature control mode is a mode where the controller controls supply of electric power to the heater such that the first temperature data is equal to a first target value. The second temperature control mode is a mode where the controller controls supply of electric power to the heater such that the second temperature data is equal to a second target value.
When a fixing operation of the print medium has been completed, the controller switches from the second temperature control mode to the first temperature control mode.
If the second temperature data becomes below a certain value during the first temperature control mode, the controller switches from the first temperature control mode to the second temperature control mode.
When continuous printing is being performed in the first temperature control mode and a number of printed pages reaches a certain value (e.g., second target value minus 15xc2x0 C.), the controller switches from the first temperature control mode to the second temperature control.
The controller controls supply of electric power to the heater in the first temperature control mode during a standby state where the image-forming apparatus waits for a print command. When the print medium has a width smaller than a reference width and the image forming apparatus receives a print command of continuous printing, the controller controls supply of electric power to the heater in the second temperature control mode.
The heater includes a first heater element having a first length and a second heater element having a second length shorter than the first length. When printing is performed on the print medium having a smaller width than a reference width, the controller supplies electric energy to the second heater element. If the first temperature data decreases below a lower limit, then the controller supplies electric energy to the second heater element and the first heater element.
When the first temperature data becomes equal to the first target value, the controller sets the second temperature data as the second target value.
The controller performs a correction operation in which the second temperature data is corrected to determine an approximate actual surface temperature of the heat roller. The controller stores a first correction value and a second correction value, the first correction value being a difference between the first temperature data and a first item of data that describes the actual surface temperature for a proper fixing operation, and the second correction value being a difference between the first temperature data and the second temperature data. When the controller is performing a printing operation, the controller adds the first correction value and second correction value to the second temperature data, thereby determining the approximate actual surface temperature of the heat roller. The controller performs a warm-up operation where when the print medium is not being passing through the fixing unit, the heat roller is rotated and the controller supplies electric power to the heater such that the actual surface temperature becomes the first item of data. The second correction value is determined immediately before the warm-up operation is halted. The controller detects an initial value (e.g., second temperature data) of a surface temperature of the heat roller after power up and before the controller supplies electric power to the heater. The controller performs the warm-up operation for a period of time in accordance with the initial value, the period of time being increased stepwise as the initial value decreases.
The second temperature detector opposes the heat roller with a gap therebetween. The controller incorporates a control program in which a size of the gap is determined based on the first temperature data, the second temperature data, and the first items of data, and then the fixing temperature is controlled in accordance with the size of the gap.
When the first temperature data exceeds a certain value during the second temperature control mode, the controller halts printing; wherein when the first temperature data decreases to a third target value after the first temperature has exceeded the certain value, the controller resumes printing.
When the first temperature data exceeds a first value during the second temperature control mode; the controller causes the print medium to be advanced at a low speed. When the first temperature data decreases to a second value after the controller has caused the print medium to be advanced at a low speed, the controller causes the print medium to be advanced at a high speed.
The controller incorporates a temperature control program in which an amplitude of ripple in the second temperature data is determined and the fixing temperature of the heat roller is corrected in accordance with the amplitude of ripple.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.